The brain's perception processes run according to a set of basic assumptions gained through experience. One of these assumptions is that there is only one "correct" way to interpret information coming from the senses. The brain's job is to go with this interpretation. Most of the time, we're unaware of the process of choice, such as when we decide that the object in front of us is a table and not a chair.

Ambiguous illusions - this way or that
Ambiguous illusions baffle the mind because they open the door to two different ways of understanding reality. The brain is not designed to handle two interpretations of the same picture. Therefore, we're unable to perceive the two interpretations simultaneously; the brain chooses one and ignores the other. In many cases, our perception switches back and forth between the two.

Which one's the real picture?
One of the brain's functions is to identify objects that have meaning. Before doing that, it needs to separate the object from its background. In many ambiguous illusions, the meaningful object according to one interpretation of the picture becomes the background in another interpretation.